(1) Field of the Invention
This invention relates to electroluminescent devices and to large area display panels such as employ uniformly illuminated surfaces to back-light graphic matter positioned thereover.
(2) Description of the Prior Art
Electroluminescent devices are generally well known, particularly as small area devices suitable for use as bedroom night-lights and the like. The development of larger area devices of several square feet or more has, for the most part, been thwarted by two factors: the devices utilize a transparent electrode which must also be sufficiently conductive so that unipotential surfaces exist when a voltage is applied to one edge of the electrode, thus enabling uniform emission/unit area throughout the device. Since such electrodes are often metallic thin-films, the conductivity of the electrode is optimized simply by making the film thicker; however, with thickness comes opacity; for a transparent electrode, the film must be as thin as possible.
Accordingly, prior art devices are generally constructed with an appropriately transparent electrode in which the conductivity is so low that an unacceptable potential drop exists across the surface if the device extends beyond a few inches from a bus bar. Such devices are, therefore, generally not larger than a few inches in diameter. While larger area devices have been proposed that utilize such bus bars extending in a grid-like fashion across the face of the device, such devices have not been well accepted, as the bus bars obscure light generated therebelow, resulting in the non-uniform emission of light.
Recently, techniques have been developed in which the transmissivity of such transparent electrodes has been improved through the use of a multiple-layer electrode in which a metal electrode of Au, Ag or Cu is sandwiched between thin-film layers of a dielectric material, thus forming antireflecting quarter-wavelength interference filters. An electroluminescent panel utilizing such a construction is disclosed and claimed in U.S. Pat. No. 4,020,389 (Dickson and Pruitt). Additional techniques have also been developed for effectively contacting the electrodes of such constructions. U.S. Pat. No. 4,066,925 (Dickson). While such constructions have enabled the exploitation of electroluminescent panels several feet long on each side, there yet exists a desire for electroluminescent panels useful in backlighting billboards and other larger area panels.